Many communication systems operate using a centralised scheduler which is responsible for allocating transmission resources to different nodes so that they are able to communicate with one another.
A typical example is the uplink of the UMTS LTE (Long Term Evolution), where the uplink transmissions from different secondary stations, also named User Equipments (UEs), are scheduled in time and frequency by the primary station of the cell, also named the evolved Node B (eNB). The primary station transmits a “scheduling grant” message to a secondary station, indicating a particular time-frequency resource for the secondary station's uplink transmission, typically around 3 ms after the transmission of the grant message. The grant message also typically specifies other transmission parameters, e.g. the data rate or power to be used for the secondary station's transmission.
In order for the primary station to issue appropriate grants, it needs to have sufficient information about the amount and type of data awaiting transmission in the buffer of each secondary station. The status of the secondary station's buffer is typically indicated by the secondary station to the primary station by means of a Buffer Status Report (BSR) procedure.
In addition the primary station sends messages to secondary stations indicating the resources being used for downlink transmissions. For efficient choice of transmission parameters such power, data rate and pre-coding dedicated to each secondary station, the primary station requires channel state information, typically comprising an indication of the SINR at the receiver or receivable data rate, and information on the transfer function and spatial characteristics of the channel. For the purpose of discussion this information can be considered as “CQI” or Channel Quality Information. CQI can be beneficial for selecting transmission parameters for ACK/NACK signals transmitted on the downlink in response to uplink packets, as well as for selecting transmission parameters for downlink data transmissions.
A problem with the BSR procedure above is that a secondary station is only permitted to transmit a BSR if it has a granted resource in which to transmit. One solution to this problem is for the secondary station to transmit a low overhead signal (an SR), which is followed by a grant of resources for a BSR. But this procedure may take some time.
Similarly, a secondary station can only transmit a CQI report if it has been granted resources. Currently in LTE these could be made available periodically, or on a one-off basis. The primary station can request a CQI report by setting a bit in a control channel message carried by the Physical Downlink Control Channel (PDCCH). The same message also grants uplink resource.
As a consequence, the granting of uplink resources specifically for BSR or CQI carries an overhead. If the resources are granted using a signalling message (e.g. on a control channel such as PDCCH), then there is an overhead due to the resources used in the downlink transmission of every signalling message.